Widespread bar code and other encoding technologies such as the universal product code (UPC) encoded on retail products, driver's licenses and other commercial or identification media rely upon predefined symbol sets defined for certain positions and sizes within labels and other materials. A traditional UPC, such as that illustrated in FIG. 1, and related codes however do not achieve a particularly high information density in terms of bits embedded per square inch, achieving on the order of 100-300 bits per square inch. This is due in one regard to the length and width of the code or symbol dimensions, which are comparatively elongated. This is also due in another regard to the limitation of the encoding technology to a black and white color scheme, in which the presence or absence of individual bits is represented by a single black or white mark or symbol.
While this encoding scheme may enhance detection robustness because the separation between coding symbols in terms of color space distance is greatest, and permit the use of relatively low-cost or low-resolution scanners because only black and white elements need to be discriminated, a price is paid in terms of information density. Simple black and white bar codes have therefore as a rule proved insufficient or impractical for transaction or identification applications which demand greater overall information content. Biometric IDs or medical insurance or information cards, for instance, may require the encoding of personal information such as an iris scan, a fingerprint image, a signature image, medical history, DNA or other information. In may applications, it is desirable to imprint that information on a comparatively compact plastic or paper card or other relatively inexpensive media, rather for instance than resort to the much more expensive solution of a smart card containing electronic intelligence. Drivers' licenses, passports or other ID media may likewise require a fairly high amount of information content, including for example color digital face photographs.
As the pixel resolution of both printing devices such as laser printers and detection devices such as handheld scanners has increased, the possibility has correspondingly arisen to enlarge both the symbol set and the color space in which bar and other symbol codes may be expressed. Printing devices, and scanning or input devices in particular have become available which are capable of close-contact optical or other scans at color depth resolutions of 8 bits (256 grayscale or color), 24, 32, 48 or greater bit depths. Enlarged color spaces combined with finer spatial resolution creates the potential for greater information density on media.
Yet encoding for example a driver's license or biometric identification card at 32 bits per pixel at 200 lines per inch using square or block symbols may still result in scanning errors from discolored paper, pixelation, rotation or other misalignment or other problems in reading the media and its symbols. Thus while information density may increase compared to single-line two-color codes, accuracy or ultimate density in cases may still be compromised or comparatively limited when using gray-scale or color encoded in square or block symbol sets. Other problems in bar code and other encoding technology exist.